The invention relates to a process for producing polycarbonate, wherein concentration of sodium-chloride-containing process waste water from the polycarbonate phase boundary process is increased by osmotic distillation.
Polycarbonates are customarily produced by a continuous process, by producing phosgene and subsequent reaction of bisphenols and phosgene in the presence of alkali metal and a nitrogen catalyst, chain stoppers and optionally branching agents in a mixture of aqueous-alkaline phase and an organic solvent in the boundary phase.

The production of polycarbonates, e.g. by the phase boundary process, is described in principle in the literature, see, e.g. in Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, John Wiley and Sons, Inc. (1964), pp. 50/51.
In the production of polycarbonates, the two-phase boundary process has been proven for many years. The process makes possible the production of thermoplastic polycarbonates in a number of fields of use such as, e.g. data carriers (CD, DVD), for optical applications or for medical applications.
Frequently, quality features which are described as important for the polycarbonate are good thermal stability and low yellowing. Less attention has been paid to date to the quality of the waste water occurring in the production of polycarbonates. In particular, pollution of the waste water with residual organics such as, e.g. residual phenols, is of importance here for any further treatment of the waste water, e.g. by a sewage treatment plant or by ozonolysis for oxidation of the residual organics. Here there have been a number of applications in which, however, predominantly methods for subsequent waste water treatment are described with the purpose of reducing the pollution by phenolic components as described, e.g., in JP 08 245 780 A, DE 19 510 063 A1, JP 03 292 340 A, JP 03 292 341 A and JP 02 147 628 A.
The pollution of the waste water with residual organics such as, e.g. with bisphenols or phenols, can be kept low by working with a high phosgene excess. However, this is undesirable for economic reasons.
In the production of polycarbonates with a reduced phosgene excess, there is the risk that not all of the bisphenol or all of the monophenol reacts to completion, passes into the waste water and pollutes the waste water. In addition there is the risk that the phase separation and the washing is made more difficult because surface-active phenolic OH groups remain in the polymer. As a result, not all of the water-soluble impurities may be extracted from the organic phase. This can in turn adversely affect the product quality.
It must still be emphasized that the production of polycarbonate of high quality by a continuous two-phase boundary process with simultaneously low pollution of the waste water according to the prior art is only possible with high phosgene excess or with phase separation problems associated with quality losses of the polycarbonate or by subsequent treatment of the waste water, as a result of which the economic efficiency of the process is reduced.
However, in these known processes, a high residual phenol value in the waste water of these processes, which can pollute the environment and can lead to an enhanced waste water problem for the sewage treatment plants, requires complex purification operations. For instance WO 03/070639 A1 describes removal of the organic impurities in the waste water by extracting with methylene chloride.
Customarily the sodium-chloride-containing solution is freed from solvents and organic residues and is then disposed of.
It is also known that the sodium-chloride-containing waste waters can be purified according to EP 1 200 359 A1 or U.S. Pat. No. 6,340,736 A by ozonolysis and said waste waters can then be used in sodium chloride electrolysis. A disadvantage of this process is the very costly ozonolysis.
According to EP 541 114 A2, a sodium chloride-containing waste water stream is concentrated by evaporation up to complete removal of the water and the remaining salt together with the organic impurities is subjected to a thermal treatment, as a result of which the organic components are decomposed. Particular preference is given to the use of infrared radiation. A disadvantage of the process is that the water must be completely evaporated, and so the process cannot be carried out economically.
According to WO 03/070639, the waste water from production of polycarbonate is purified by extraction and then supplied to the sodium chloride electrolysis. However, only a maximum 14% by weight of the sodium chloride from the waste water of the polycarbonate production can be recycled into the NaCl electrolysis, since in the case of larger amounts of NaCl-containing waste water, the water introduced into the electrolysis together with the NaCl-containing waste water would bring the water balance of the sodium chloride electrolysis out of equilibrium.
The sodium chloride-containing solutions which occur in the production of polycarbonate typically have a sodium chloride content of 6 to 10% by weight. Therefore, all of the sodium chloride present in the solutions cannot ever be recycled to chloride and sodium hydroxide solution in the NaCl electrolysis. At a sodium chloride concentration of 10% by weight, in the standard sodium chloride electrolysis using a commercially conventional ion-exchange membrane which exhibits water transport of 3.5 mol of water per mol of sodium, only the use of approximately 13% of the sodium chloride from the sodium chloride-containing solutions succeeds. Increase of concentration up to a saturated sodium chloride solution of approximately 25% by weight would yield a recycling rate of 38% of the sodium chloride present in the sodium chloride-containing solution. Complete recycling of the sodium chloride-containing solution is currently unknown.
Processes for raising concentration by removing water from the alkali-metal-chloride-containing waste water are known.
According to WO 01/38419, concentration of the sodium chloride-containing solution is increased by evaporation by means of a thermal process, in such a manner that a highly concentrated sodium chloride solution can be fed to the electrolysis cell. However, the evaporation is energy-intensive and costly.
Also, for example, reverse osmosis or, particularly preferably, membrane distillation or membrane contactors can be used (see MELIN; RAUTENBACH, Membranverfahren [Membrane processes]; SPRINGER, BERLIN, 2003). A disadvantage in this case is the high energy requirement to overcome the high osmotic pressures, as a result of which the process is no longer economically efficient.
The above integrated processes all have the disadvantage that, in combination with polycarbonate production only, NaCl solutions with a limited concentration (6-10% by weight) are fed to the electrolysis, and so reutilization of NaCl is only possible to a restricted extent or the concentration is energy-intensive and expensive.
Proceeding from the above-described prior art, the object is to provide a polycarbonate production process in which, during the recycling of the waste water, the reaction of sodium chloride to form chlorine and sodium hydroxide solution and possibly hydrogen can proceed with minimum energy use and therefore in a particularly economic and resource-saving manner. In addition, a process should be provided which yields products in high purity and good yield and makes possible a reduction of the environmental pollution and/or the waste water problems in sewage treatment works.
The object is achieved in that, in the process, sodium chloride-containing waste water phases are utilized by an upstream increase of concentration using an osmotic membrane distillation of the NaCl solution from the polycarbonate production for the electrolysis.